Semiconductor lasers having a Fabry-Perot resonator (hereinafter referred to as “FP-LDs”) have been known in the art. See, for example, Hiroo Yonezu, “Optical Communications Device Engineering: Light Emitting Devices And Photodetectors,” seventh edition, Kougakutosho Ltd., May 20, 2003. This nonpatent document describes the principle of laser oscillation, or lasing, in FP-LDs (pages 164-171). This nonpatent document describes longitudinal mode oscillation in FP-LDs (pages 243-251). The conditions for longitudinal mode oscillation in a FP-LD are determined by the resonator modes that overlap with the gain spectrum peak of the FP-LD, as shown in FIG. 4.2(d) on page 166 of the document. The longitudinal mode spacing of a FP-LD is typically approximately a few angstroms (see page 244 of the above document). This means that in the FP-LD, laser oscillation occurs in the several longitudinal modes that overlap with the gain spectrum peak. The gain spectrum of a FP-LD is determined by the band structure of the semiconductor material of the active layer. Since this band structure changes with temperature, so does longitudinal mode oscillation in the FP-LD. That is to say, the longitudinal mode oscillation has a temperature dependence.
Incidentally, in a FP-LD, coating layers are formed on the emitting facets (i.e., the front and rear facets) of the resonator to protect these facets (see the above nonpatent document). The reflectance Rf of the coating layer on the front facet and the reflectance Rr of the coating layer on the rear facet are set such that the FP-LD has desired characteristics. These reflectances Rf and Rr greatly affect the slope efficiency (SE) of the FP-LD, as can be seen from equation (1) discussed later. Therefore, it is ensured that the coating layers have a robust design to prevent their reflectances Rf, Rr from varying with wavelength. For example, in the case of 0.8 μm LDs for exciting, or pumping, a Nd:YAG laser, which must operate at high power, the reflectances Rf and Rr are usually approximately 10-20% and 95-100%, respectively.
There will now be described the temperature dependence of the oscillation wavelength of a conventional FP-LD in which a coating layer is formed on each facet. In this conventional FP-LD, the reflectance Rf of the coating layer on the front facet is 12% and the reflectance Rr of the coating layer on the rear facet is 99%. These reflectances do not have a temperature dependence. Further, the internal loss αi of the resonator is 1 cm−1 and the length Lc of the resonator is 1 mm.
As shown in FIG. 12, the (overall) loss α of the FP-LD is constant (11.7 cm−1) independent of the wavelength of the laser beam. In FIG. 12, symbol “GS(T1)” indicates the gain spectrum of the FP-LD at operating temperature T1 when a current is injected into the FP-LD. In this case, laser oscillation occurs at around wavelength λL(T1) at which the loss α is balanced by the gain (or gain spectrum peak) in the FP-LD. This oscillation is in a plurality of longitudinal modes whose wavelengths are centered around the wavelength λL(T1) at which the gain is peaked.
The forbidden band gap of the semiconductor material of the FP-LD decreases as the temperature of the FP-LD increases. In FIG. 12, symbols “GS(T2)” and “GS(T3)” indicate the gain spectra of the FP-LD at operating temperatures T2 and T3, respectively (T1<T2<T3). As shown in FIG. 12, increasing the temperature of the FP-LD results in a shift of its gain spectrum toward longer wavelengths. As a result, the wavelength λL at which the loss α is balanced by the gain is also shifted toward longer wavelengths, as indicated by symbols “λL(T1)”, “λL(T2)”, and “λL(T3)” in FIG. 12.
It should be noted that table 4.2 on page 244 of the nonpatent document noted above shows the degrees of temperature dependence of the oscillation wavelengths of conventional FP-LDs. For example, the oscillation wavelength of a 0.8 μm LD formed of AlGaAs material has a temperature dependence of approximately 2-3 angstroms/° C.
Other documents are Japanese Laid-Open Patent Publication Nos. H5-82897 (1993), H9-107156 (1997), 2005-72488, and 2004-111622.
In order to use a FP-LD as an excitation source for a solid laser such as a Nd:YAG or Yb:YAG, it is necessary to accurately control the oscillation wavelength of the FP-LD. Specifically, the oscillation wavelength of the FP-LD must be adjusted to within the range of 805-811 nm, preferably 807-809 nm, when it is applied to the Nd:YAG. On the other hand, the oscillation wavelength must be adjusted to within the range of 931-949 nm, preferably 937-943 nm, when the FP-LD is applied to the Yb:YAG.
It should be noted that the above solid lasers are primarily used for machining, such as welding, cutting, boring, and soldering. Such machining requires high power (a few watts to a few tens of kilowatts), and the (energy) conversion efficiency from the excitation light to the laser light is high (a few tens of percent). This means that exciting a solid laser requires a few to a few hundreds of FP-LDs that deliver a power on the order of a few tens of watts and that have substantially equal oscillation wavelengths. In order to achieve a laser power output on the order of a few tens of watts, a few tens of broad area FP-LDs may be integrated together into a LD bar having a width of 1 cm and mounted on a water-cooled microchannel, as is known in the art. Bookham, Inc. provides such packages (hereinafter called “LD packages”), namely BAC50C-806-01, BAC50C-806-02, etc.
Equalizing the oscillation wavelengths of LD packages requires reducing the variation in the composition and in the thickness of the active layers of the FP-LDs in each LD bar. Further, it is also necessary to reduce the assembly variations of the LD bars and reduce the variation in the thermal resistance of the packages. Further, the temperature of the cooling water for the LD packages must be accurately controlled on the solid laser side, which requires a chiller having a large cooling capacity.
On the other hand, it is desirable to reduce the variation in the oscillation wavelengths of the FP-LDs with variations in the composition and in the thickness of the active layers of the FP-LDs and with variations in the thermal resistance of the packages. That is, it is desirable to reduce the temperature dependence of the oscillation wavelengths of the FP-LDs to improve the yield of the LD packages.